Authorities said that the shooter, a 49-year-old German man, told officers he acted “out of anger” after the 20-year-old clerk would not let him buy a beer.
“He further stated during interrogation that he rejected the measures against the coronavirus,” the Trier police department said in a statement.
Among other measures to stop the spread of the virus, Germany requires people to wear masks in stores.
Police said the suspect left the Idar-Oberstein gas station after the dispute then returned a half-hour later wearing a mask and fatally shot the clerk in the head.
The suspect, a German citizen who wasn’t identified by name in line with privacy laws, fled the scene and turned himself in to police on Sunday.
The shooting prompted warnings from senior politicians against the radicalization of people who oppose the country’s pandemic restrictions.
The center-left Green Party’s candidate to succeed German Chancellor Angela Merkel voiced dismay at the killing. Germany’s federal election is scheduled for Sunday.
“I’m shaken by this terrible murder of a young man who merely asked that existing rules be followed,” Annalena Baerbock said in a tweet.
She also expressed concern about the radicalization of Germany’s Querdenken movement, which includes people who oppose masks and vaccines, conspiracy theorists, and some far-right extremists.
Facebook last week removed almost 150 accounts and pages linked to the movement under a new policy focused on groups that spread misinformation or incite violence but which didn’t fit into the platform’s existing categories of bad actors.
Authorities didn’t immediately say whether the suspect in the gas station killing was associated with the movement, which has come under increasing scrutiny from Germany’s security services following a series of large anti-government protests.
Prosecutors told Germany’s dpa news agency that the man wasn’t previously known to police and that he wasn’t legally entitled to possess the firearm found at his house.
Paul Ziemiak, the general secretary of Merkel’s center-right Christian Democratic Union Party, called the clerk’s slaying “incomprehensible.”
“A young man was virtually executed because he pointed out the mask requirement,” Ziemiak said on Twitter. “An inconceivable level of radicalization!”
The Associated Press contributed to this report.